U.S. patent number 4,885,564 [Application Number 07/189,566] was granted by the patent office on 1989-12-05 for power line carrier communication system for monitoring refrigerated containers.
This patent grant is currently assigned to Thermo King Corporation. Invention is credited to Arthur A. Anderson, Laurence R. Brickner, David A. Christiansen, Robert G. Colclaser III, Dannis R. Johnson, John G. Leddy, Leonard C. Vercellotti.
United States Patent |
4,885,564 |
Vercellotti , et
al. |
December 5, 1989 |
Power line carrier communication system for monitoring refrigerated
containers
Abstract
A power line carrier communication system for monitoring
refrigerated containers which includes a master monitoring unit and
a first power line interface which interchange messages in a first
format. The first power line interface translates the first format
to a second format suitable for power line environment, and
messages in the second format are applied to a power line. Remote
monitoring units receive the messages from the power line, and they
return messages to the power line containing status data relative
to refrigerated containers. The second format includes a message
starting preamble having a duration and logic level which is not
duplicated by normal operation of the apparatus, enhancing the
probability of proper message synchronization and reception over
noisy power line environments.
Inventors: |
Vercellotti; Leonard C.
(Oakmont, PA), Anderson; Arthur A. (Irwin, PA),
Christiansen; David A. (Bloomington, MN), Johnson; Dannis
R. (Savage, MN), Brickner; Laurence R. (Marathon,
NY), Colclaser III; Robert G. (North Huntingdon, PA),
Leddy; John G. (Lexington, MA) |
Assignee: |
Thermo King Corporation
(Minneapolis, MN)
|
Family
ID: |
22697886 |
Appl.
No.: |
07/189,566 |
Filed: |
May 3, 1988 |
Current U.S.
Class: |
370/471;
340/12.37; 340/3.5; 340/310.16; 340/538 |
Current CPC
Class: |
F25D
29/008 (20130101); G05D 23/1905 (20130101); H04L
29/06 (20130101) |
Current International
Class: |
H04L
29/06 (20060101); G05D 23/19 (20060101); F25D
29/00 (20060101); H04M 011/04 () |
Field of
Search: |
;340/31R,31A,825.07
;364/551.01 ;307/39,40 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Orsino; Joseph A.
Assistant Examiner: Tumm; Brian R.
Claims
We claim as our invention:
1. A power line carrier communication system for monitoring
refrigerated containers, comprising:
a local electrical distribution power line,
a master monitoring unit,
a first power line interface between said master monitoring unit,
and said local electrical distribution power line,
a plurality of refrigerated containers,
and a plurality of remote monitoring units, one for each
refrigerated container to be monitored by said master monitoring
unit,
said master monitoring unit preparing and transmitting messages to
said first power line interface in a first message format,
said first power line interface including means for transmitting
messages to and receiving messages from said master monitoring
unit, message translating means, and means for applying messages to
and receiving messages from said local electrical power
distribution line,
said message translating means translating messages received from
the master monitoring unit which are to be applied to the local
electrical distribution power line into a second format, and
translating messages received from the local electrical
distribution power line to the first message format,
each of said remote monitoring units including means for receiving
said second format messages from said local electrical distribution
power line, and processing means for processing said second format
messages, with said processing means including means for obtaining
status information from an associated refrigerated container, means
for incorporating said status information into a message having the
second message format, and means for applying said status message
to the local electrical distribution power line.
said second format including message information which includes a
predetermined periodic logic level for at least one bit of every
ten message bits, and a message starting preamble which includes
the same logic level for at least ten consecutive bits, with this
ten bit message starting logic level being different than said
predetermined periodic logic level,
said first message format including a field position which
indicates type of assignment to be performed by the first power
line interface in messages received from the master monitoring
unit, and wherein the first power line interface uses said same
field position to indicate assignment success or failure in
messages sent to the master monitoring unit.
2. The power line carrier communication system of claim 1 wherein
message information in both the first and second formats includes
packets of characters, with each character having one start bit,
eight data bits, and one stop bit, and wherein dead time between
any two consecutive characters in the second format is less than
one-half of a bit, to prevent a delay within a message from
emulating the message starting preamble of the second format.
3. The power line carrier communication system of claim 1 wherein
the first format includes a start byte followed by a two byte
unsigned integer which indicates the packet length of the following
message.
4. The power line carrier system of claim 1 wherein the plurality
of remote monitoring units have a universal address to which all
will respond, and each of the remote monitoring units have a unique
address to which only it will respond.
5. A power line carrier communication system for monitoring
refrigerated containers, comprising:
a local electrical distribution power line,
a master monitoring unit,
a first power line interface between said master monitoring unit
and said local electrical distribution power line,
a plurality of refrigerated containers,
a plurality of remote monitoring units, one for each refrigerated
container to be monitored by said master monitoring unit,
said master monitoring unit preparing and transmitting messages to
said first power line interface in a first format,
said first power line interface including means for receiving
messages from said master monitoring unit, means for translating at
least certain of the messages into a second format, and means or
applying said second format messages to said local electrical power
distribution line,
each of said remote monitoring units including means for receiving
said second format messages from said local electrical distribution
power line, and means for processing said second format
messages,
said second format including message information which includes a
predetermined periodic logic level for at least one bit of every
ten message bits, and a message starting preamble which includes
the same logic level for at least ten consecutive bits, with this
ten bit message starting logic level being different than said
predetermined periodic logic level,
a remote electrical distribution power line having refrigerated
containers connected thereto to be monitored by the master
monitoring unit, remote radio means at the remote electrical
distribution power line,
and local radio means for transmitting messages initially prepared
by the master monitoring unit to said remote radio means,
said messages transmitted by said local radio means being in the
second format, with said second format including an address field
which identifies an electrical distribution power line the message
is intended for.
6. The power line carrier communication system of claim 5 wherein
the remote monitoring units will only accept a message which has an
address tag which identifies the local electrical distribution
power line, with the first power line interface applying a second
format message to the local electrical distribution power line
without changing the address tag, and wherein the remote radio
means changes the address tag in a second format message to the
address of the local electrical distribution power line before
applying a message to the remote electrical distribution power
line, enabling refrigerated containers to be connected to any
electrical distribution power line without modification.
7. A power line carrier communication system for monitoring
refrigerated containers, comprising:
a local electrical distribution power line,
a master monitoring unit,
a first power line interface between said master monitoring unit
and said local electrical distribution power line,
a plurality of refrigerated containers,
and a plurality of remote monitoring units, one for each
refrigerated container to be monitored by said master monitoring
unit,
said master monitoring unit preparing and transmitted messages to
said first power line interface in a first format,
said first power line interface including means for transmitting
messages to and receiving messages from said master monitoring
unit, message translating means, and means for applying messages to
and receiving messages from said local electrical power
distribution line,
said message translation means translating messages received from
the master monitoring unit which are to be applied to the local
electrical distribution power line into a second format, and
translating messages received from the local electrical
distribution power to the first message format,
each of said remote monitoring units including means for receiving
said second format messages from said local electrical distribution
power line, and processing means for processing said second format
messages, with said processing means including means for obtaining
status information from an associated refrigerated container, means
for incorporating said status information into a message having the
second message format, and means for applying said status message
to the local electrical distribution power line,
said second format including message information which includes a
predetermined periodic logic level for at least one bit of every
ten message bits, and a message starting preamble which includes
the same logic level for at least ten consecutive bits, with this
ten bit message starting logic level being different than said
predetermined periodic logic level,
each of said remote monitoring units including a first
computer,
said means for obtaining status information in certain of the
remote monitoring units including first status determining means,
and the means for obtaining status information in certain other
remote monitoring units including second status determining means,
with the first status determining means including a refrigeration
controller which includes a second computer, and with the second
status determining means including direct connections to the
refrigerated container which provide a plurality of status signals,
with the first computer reading the status signals provided by said
direct connections.
8. A power line carrier communication system for monitoring
refrigerated containers, comprising:
a local electrical distribution power line,
a master monitoring unit,
a first power line interface between said master monitoring unit
and said local electrical distribution power line,
a plurality of refrigerated containers,
a plurality of remote monitoring units, one for each refrigerated
container to be monitored by said master monitoring unit,
said master monitoring unit preparing and transmitting messages to
said first power line interface in a first format,
said first power line interface including means for receiving
messages from said master monitoring unit, means for translating at
least certain of the messages into a second format, and means for
applying said second format messages to said local electrical power
distribution line,
each of said remote monitoring units including means for receiving
said second format messages from said local electrical distribution
power line, and means for processing said second format
messages,
said second format including message information which includes a
predetermined periodic logic level for at least one bit of every
ten message bits, and a message starting preamble which includes
the same logic level for at least ten consecutive bits, with this
ten bit message starting logic level being different than said
predetermined periodic logic level,
and radio means for transmitting message prepared by said first
power line interface to a remote power line carrier communication
system associated with a remote electrical distribution power line
having refrigerated containers connected thereto to be monitored by
the master monitoring unit,
said radio means transmitting messages in the second message
format.
9. The power line carrier communication system of claim 8 wherein
the messages prepared by the first power line interface for the
radio means were initiated by the master monitoring unit.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to the monitoring of refrigerated
containers, and more specifically to monitoring refrigerated
containers over electrical distribution power lines.
2. Description of the Prior Art
Electrical utilities have used their high voltage transmission
lines for many years for communication with remote switching and
substation sites, for supervisory control purposes. Transmission
lines are ideal for communication as they extend from the power
generation site to the remote sites without intervening obstacles.
Use of the distribution power lines, however, has been slower to
develop, as the distribution power lines are susceptible to
electrical noise and interference, and they include distribution
transformers, electrical loads, sectionalizing switches, capacitor
banks, and the like, which attenuate communication frequencies. The
increasing desirability of being able to selectively and remotely
control electrical loads on the distribution system, and the
availability of low cost encoders for automatic meter reading, have
produced a flurry of activity in the use of distribution power
lines for communication purposes. U.S. Pat. Nos. 3,911,415;
3,942,168; 3,942,170; 3,967,264; and 3,980,954, describe some of
the early problems encountered, and solutions thereto, when using
electrical distribution power lines for communication.
A specialized use of electrical distribution power lines for
communication purposes has been disclosed in U.S. Pat. No.
4,234,926. Refrigerated containers, called "reefers" are monitored
by a central computer, using the power lines for polling or
interrogating remotely located computerized monitoring units
associated with the reefers, as well as for receiving reefer status
data in return. Since large numbers of reefers are stacked aboard
ships, as well as in ship terminals, the use of the electrical
power lines connected to the reefers for the additional functions
of monitoring and control substantially increases the speed and
reliability of the monitoring process, which is normally manually
performed.
While the use of ship and ship terminal electrical distribution
lines for communication may seem simple and straight forward
compared with the attenuating obstacles encountered by the electric
utilities in using their distribution systems for communication,
ship and ship terminal electrical systems can have substantial
amounts of electrical noise. For example, large adjustable speed
motor drives which chop the electrical waveform may be used, which
feed large amounts of electrical noise back into the power
lines.
SUMMARY OF THE INVENTION
Briefly, the present invention increases the reliability of power
line carrier communications for monitoring refrigerated containers
aboard ships and in ship terminals by making it easier for the
communication system to detect the start of a message received over
the power line. A master monitoring unit (MMU) having a computer
directs the monitoring and control process, providing messages in a
first format for a first power line interface also referred to as a
network central control unit (NCCU). The NCCU translates the
messages to a second format more suitable for the power line
environment, it modulates the message, such as by frequency shift
keying (FSK), and it places the translated messages on the
electrical distribution power line. Remote monitoring units (RMUs)
gather status information from the reefer's refrigeration units.
The RMUs include second power line interfaces. The second power
line interfaces are coupled to the power line and they demodulate
the high frequency messages.
The RMUs sample the incoming line at a rate greatly in excess of
the message bit rate, looking for a valid message starting
preamble. The message itself has a predetermined logic level, such
as a logic zero, in every ten message bits. The message starting
preamble has at least ten consecutive bits at a logic level
opposite to the predetermined logic level which must periodically
occur in every ten message bits. If this periodic logic level is a
logic zero, then the starting preamble must be at the logic one
level for at least ten bit times. Since ten logic ones cannot occur
during normal message composition and transmission, the chances of
detecting the valid start of a message are greatly enhanced, and
the chances of false starts once a message begins are
minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be better understood and further advantages and
uses thereof more readily apparent when considered in view of the
following detailed description of exemplary embodiments, taken with
the accompanying drawings, in which:
FIG. 1 is a functional block diagram of a power line carrier
communication system for monitoring refrigerated containers
according to the teachings of the invention;
FIG. 2 illustrates a first message format, used between a master
monitoring unit and a first power line interface;
FIG. 3 illustrates a second message format, used on the electrical
distribution power line;
FIG. 4 illustrates normal message composition in which a logic
zero, must appear in every ten bits;
FIG. 5 is a block diagram of a power line carrier communication
system constructed according to the teachings of the invention,
illustrating message flow, and the locations of the two different
message formats in the system; and
FIG. 6 is a detailed block diagram of the power line carrier
communication system shown in FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings, and to FIG. 1 in particular, there
is shown a functional block diagram of a power line carrier
communication system 10 constructed according to the teachings of
the invention. System 10 includes a plurality of transportable
refrigerated containers or reefers, such as reefers 12 and 13.
Reefer 12 includes a refrigeration unit 14 which conditions the air
in a container 15, and reefer 13 includes a refrigeration unit 16
which conditions the air in a container 17. U.S. Pat. Nos.
4,402,191; 4,409,797; and 4,424,684, which are assigned to the same
assignee as the present application, illustrate typical container
refrigeration units.
Each reefer includes one of two different types of remote
monitoring units, depending upon whether the reefer already has a
computer based refrigeration controller, such as controller 18
associated with reefer 12. Controller 18 may be the refrigeration
controller disclosed in U.S. Pat. No. 4,663,725 entitled
"Microprocessor Based Control System And Method Providing Better
Performance And Better Operation Of A Shipping Container
Refrigeration System", which is assigned to the same assignee as
the present application. This patent is hereby incorporated into
the specification of the present application by reference.
Reefer 12, having a refrigeration controller 18, requires a remote
monitoring unit 19, referred to as an integrated remote monitoring
unit (IRMU). If the reefer does not have a controller 18, such as
reefer 13, the RMU will include a data acquisition function which
obtains status information directly from a refrigeration unit, and
this type of RMU is referred to as a stand alone remote monitoring
unit (SRMU) 20. IRMU 19 and SRMU 20 will be referred to
collectively as "RMUs" when the distinction between them is not
pertinent to what is being discussed.
Reefers 12 and 13 are monitored by a master monitoring unit (MMU)
22 which includes a central computer for preparing, transmitting
and receiving messages. A network central control unit (NCCU) or
first power line interface 24 is disposed between MMU 22 and an
electrical distribution power line 26 which extends to the
locations of the plurality of reefers 12 and 13. The communications
between MMU 22 and NCCU 24 are not over power line 26, and thus a
first message format is used which may be formulated with minimal
concern with electrical noise. Thus, a generally accepted
communications medium, such as an RS-232-C standard may be applied.
NCCU 24 includes a transmitter/receiver 28 for receiving messages
in the first format, translating means 30 for translating the first
message format to a second message format tailored for the severe
power line environment, a power line carrier transceiver (CCT) 31
for modulating the message, and power line coupling means 32 for
applying the modulated message to power line 26.
IRMU 19 includes power line coupling means 34, a CCT 36 for
demodulating messages received from power line 26, and computer
means 38 for receiving messages from MMU 22 and for preparing and
sending messages to MMU 22 when required. For example, if the
message requested the status of reefer 12, IRMU 19 would send
status information obtained from refrigeration controller 18 back
to MMU 22. IRMU 19 would prepare the message in the second format
in computer 38. CCT 36 modulates the message, and power line
coupling means 34 applies it to power line 26. NCCU 24 detects the
message via power line coupling means 32, CCT 31 demodulates the
message, translating function 30 translates the message from the
second to the first message format, and transmitter/receiver 28
sends the reformatted message to MMU 22.
SRMU 20 includes a power line coupling 34', a CCT 36', and a
computer 38', all similar to these functions in IRMU 19. SRMU 20
further includes means 40 for obtaining data available in digital
form from refrigeration unit 16, and means 41 for obtaining data
available in analog form from refrigeration unit 16.
A message format which may be used for the first format is shown in
FIG. 2. The first format, referenced 42, will first be described as
it is prepared by MMU 22. The first message format 42 may start
with an optional synchronization field 43 of any desired number of
bytes, followed by a one byte start-of-text (STX) character 44
which delimits the start of a valid message. NCCU 24 strips the
synchronization field 43, if used, from the start of a message.
When a valid start byte 44 is detected, the next two bytes 45 are
used to define an unsigned integer which indicates the packet
length in bytes of the following data. The following data includes
a one byte task number 46 assigned by MMU 22, which is an
identification number used only by MMU 22. The identification
number is followed by a one byte character 47 which informs NCCU 24
as to the message type, ie., the type of processing required. For
example, the message may be directed only to NCCU 24, such as
setting up internal configurations; or it may be a message which is
to be applied to power line 26 directed to all of the the RMUs, or
to a specifically addressed RMU. A variable length data field 48
follows the processing type identifier 47, with its contents
depending upon the type of message to be processed. For example, if
the message is to be applied to power line 26, the data field would
contain an address tag, a universal or a specific RMU address, plus
data relative to the type of command being sent.
When the communication is between NCCU 24 and a local power line,
the address tag must be set to a predetermined value before the
RMUs will respond to any message. For purposes of example, it will
be assumed that the predetermined value is an ASCII "A" (41H). The
address tag allows expansion of the monitoring system to include
remote power lines, by using the address tag to address one or more
remote power lines which are communicated with by radio. When an
interface associated with a remote power line is addressed by the
address tag, the remote interface will swap an ASCII "A" for the
value in the address tag before applying the message to the
associated power line. Thus, all reefers, regardless of which power
line they are connected to will automatically be set up to respond
to messages from MMU 22.
A two byte error check field 49 follows, such as a field generated
by using the cyclic redundancy check (CRC), ie., a polynomial
calculation performed on the message data bits.
The first message format 42 when prepared by NCCU 24 for
transmission to MMU 22 includes the optional sync field 43, the
starting character 44, and the two byte packet length 45. The task
number used in field 46 is the same task number previously assigned
to the MMU request for which this message is a response. NCCU 24
uses the one byte field 47 used by MMU 22 to indicate type of
message, to indicate success or failure in implementing the command
in the message received from MMU 22. The data field 48 includes the
data resulting from the MMU command, e.g., RMU status data,
followed by the CRC error checking field 49.
FIG. 3 indicates the second message format 51, which is used for
power line communications. A message starting preamble 53 includes
at least three transition changes, required to synchronize
communications between carrier receivers "listening" to the power
line 26. The transitions are followed by a predetermined logic
level which persists for at least the length of time required for
10 message bits. The message following the starting preamble 53
consists of a power line prefix 55, 57 and 59 in ASCII, and a data
field 61 in non-ASCII characters, controlled in ANSI X3.28 format.
Since each valid character consists of framing bits surrounding a
byte of information, valid characters cannot be formed in either
ASCII or non-ASCII which do not have a logic zero in every ten bits
of a continuous message. The ten continuous bits of a predetermined
logic level in the starting preamble have a logic level opposite to
the periodic logic zero, and they are thus at the logic one
level.
The ten logic one bits in the starting preamble 53 must be detected
before any message will be received. As hereinbefore stated, the
formats of the ASCII and non-ASCII characters do not allow ten
logic ones to be valid characters and they cannot occur within a
message if the message is transmitted continuously. FIG. 4 sets
forth the format 65 of a character byte. Format 65 has a low start
bit, eight data bits BO through B7, and a high stop bit. Thus, it
is important that in the second message format that there be no
delay between bytes of a packet which exceeds one-half bit, as the
line will be held high while awaiting more data. For example, if
the eight data bit locations are high, the stop bit will make nine
high bits in a row. A delay of one bit at this point will delay the
normal low start bit and emulate a valid starting preamble 53.
Since the starting preamble 53 cannot be transmitted by normal UART
operation, the starting preamble will not interfere with higher
level protocols, as would be the case if the starting preamble
would require synchronizing on the reception of a particular valid
asynchronous character.
The ten logic one bits also have the advantage of flushing the
receiver's character buffer by allowing it to finish any character
reception in progress and guarantee no new character will be
received until the first character of the next valid message
As hereinbefore stated, the valid starting preamble 53 is followed
by fields 55, 57 and 59 collectively referred to as the power line
prefix. Field 55 is used for the one byte message type, as
prescribed by the message-type byte 47 described relative to the
first message format 42 used between MMU 22 and NCCU 24. A reply
message from an RMU will also use this same byte character in field
55.
Field 57 contains an address tag byte which is the same as that
provided in the first byte of the data field 48 of the first
message format 42. As hereinbefore stated the address tag is set to
an ASCII "A" in communications between NCCU 24 and a local power
line; and to a unique value recognized by power line carrier
systems associated with remote power lines, when the monitoring
system includes a radio link for communicating with one or more
remote power lines.
Field 59 consists of ten bytes which define an address field. The
address placed in field 59, which is the same address contained in
the data field 48 of the first format 42, may be an address unique
to a specific RMU, or it may be a universal address recognized by
all RMUs. A universal address is used during system reset, and in
the process of initially giving a RMU an unique address. When MMU
22 is polling for reefer status data, it will insert a specific RMU
address.
The one byte message type 55, the one byte address tag 57, and the
ten byte RMU address 59, collectively called the power line prefix,
are all in ASCII characters.
The next field 61, variable in length, is the data field, and it
contains data in non-ASCII characters, including control characters
in ANSI X3.28 format. It will contain the specific command or task
to be performed by an RMU. As hereinbefore stated, FIG. 4 indicates
the format 65 of all characters, including prefix and data, which
format includes a low start bit, eight data bits, and a high stop
bit. Thus, there will always be at least one logic zero, i.e., the
start bit, in every ten bit data character. The message ends with a
two byte CRC error check field 63.
When a RMU responds to a command from MMU 22, it will prepare a
message in the second format 51 just described, inserting the data
requested by the specific MMU command in the data field 61.
FIG. 5 is a more detailed block diagram of a power line carrier
communication system 70 which utilizes the same reference numerals
as FIG. 1 where appropriate. NCCU 24 receives messages in the first
format 42 and translates them to the second format 51. When the
message is for RMUs connected to the local power line 26, MMU22
will have inserted an ASCII "A" into the address tag field 47 of
the first format 42, which tells NCCU 24 that the message is for
the local power line 26. NCCU 24 will maintain the ASCII "A" in the
second format, appearing in field 57, and it will apply the message
to power line 26.
When one or more remote power lines, such as remote power line 72,
include reefers to be monitored, messages in the second format 51
are obtained from NCCU 24 and sent by radio link 74 to a remote
power line interface. Radio link 74 includes a local central
control unit (LCCU) 76 which includes a radio transmitter/receiver.
Each remote power line, such as remote power line 72, includes a
remote central control unit (RCCU) 78 which also has a
transmitter/receiver. NCCU 24 translates messages received from MMU
22 from the first message format 42 to the second message format
51. The address tag located in field 48 of the first message format
42 an in field 57 of the second message format 51 provides a
predetermined power line identification, which, as hereinbefore
stated, will be assumed to be an ASCII "A" for the local power
line. Thus, an ASCII "B", for example, may be used for messages
destined for a first remote power line 72, etc. All RMUs,
regardless of which power line they are connected to will only
respond to messages which have an ASCII "A" in the address tag 57.
If the message received by NCCU 24 from MMU 22 has an ASCII "A" in
the address tag field 48, NCCU 24 simply applies the message to the
local power line 26. If it has an identification other than an
ASCII "A", LCCU 76 transmits the second formatted message by radio
to all remote power lines. Each RCCU 78 receives the broadcast
message, and if the address tag bears its unique identifier, the
addressed RCCU 78 swaps its identifier for an ASCII "A" in the
address tag field 57 and applies the message to its associated
remote power line 72. Responses by the RMUs connected to the remote
power line 72 are received by RCCU 78 which then swaps its
identifier for the ASCII "A" in the address tag field 57 and
broadcasts the message back to the LCCU 76, which directs the
message to MMU 22 via NCCU 24. Thus, all RMUs, regardless of which
power line they are connected to will see an ASCII "A" in the
address tag field 57, and reefers and RMUs may be connected to any
power line without modification.
FIG. 6 is a detailed block diagram illustrating an exemplary
embodiment of power line communication system 10 shown in FIG. 1.
NCCU 24 includes an RS-232 transmit/ receive chip 90, such as
Maxim's MAX231, an asynchronous communications interface adapter
(ACIA) chip 92, such as RCA's CDP65C51, a microcomputer 94 having
an internal UART 95, such as Intel's 80C31, a chip select chip 96,
an address latch 98, a random access memory (RAM) 100, an erasable
programmable read-only memory (EPROM) 102, and the hereinbefore
mentioned carrier current transceiver (CCT) 31, such as National
Semiconductor's LM1893. The power line coupling means 32 for
applying messages to, and receiving messages from, power line 26
includes a coupling transformer 106 having a winding connected to
CCT 31 and a winding connected to the three phases and neutral of
distribution power line 26 via coupling capacitors 108, 110 and
112.
A message prepared by MMU 22 is sent to RS-232 chip 90 and then to
ACIA 92, with ACIA 92 being a program controlled interface between
microcomputer 94 and the serial data provided by chip 90.
Microcomputer 94 reformats the message from the first to the second
formats, and the internal UART 95 sends the message to CCT 31 which
FSK modulates the message and applies it to power line 26 via the
coupling transformer and coupling capacitors broadly referenced 32,
as they perform function 32 shown in block form in FIG. 1.
IRMU 19 includes coupling capacitors and a coupling transformer,
broadly referenced 34, as they perform function 34 shown in block
form in FIG. 1. A CCT chip 36 similar to chip 31, is connected to a
microcontroller chip 114 which includes an internal UART 116. An
ACIA chip 118, along with optoisolators 120, interface IRMU 19 with
refrigeration controller 18. A chip select chip 122, an address
latch 124, an EPROM 126, and a RAM 128, are also included, all of
which may be similar to the chips in NCCU 24. In addition, IRMU 19
includes a RAM 130 backed by a battery 132. RAM 130 contains an
image of the latest status information provided by controller 18
for rapid dumping to the power line 26 when requested by MMU 22,
and it also includes a maintenance record of the reefer 12 it is
associated with, downloaded by MMU 22. IRMU 19 also includes a
nonvolatile RAM 134, which contains the unique address of the IRMU
19.
SRMU 20 does not require a computerized refrigeration monitor and
controller 18 for data gathering purposes. Functions in SRMU 20
which may be the same as just described for IRMU 19 are given like
reference numerals with a prime mark, and will not be described
again. The data gathering function 40 shown in FIG. 1 for obtaining
data from refrigeration unit 16 which is available in digital form
is provided by a digital I/0 chip 136 which includes input status
and an output latch under the control of computer 114'. The data
gathering function 41 shown in FIG. 1 for obtaining data from
refrigeration unit 16 available in analog form, such as data from
temperature sensors, includes a multiplexer 138, such as RCA's
4051, which is addressed by an address latch 140 which is under the
control of computer 114'. The output of multiplexer 138 is
connected to computer 114'via an A/D converter 142. In the SRMU 20
the ACIA function 118'is connected to a plug-in receptacle 144 via
an RS'232 chip 146 for optional communication with a hand held data
retriever (not shown).
In addition, SRMU 20 also includes a battery and charger (not
shown) so that data may be logged from reefer 13 by SRMU 20 without
being connected to power line 26, and a timer 148 is provided to
periodically provide signals which initiate the retrieval of status
information from refrigeration unit 16.
* * * * *